A conceptual analysis of differential item functioning in terms of a multidimensional item response model

Abstract

Differential item functioning (DIF) has been informally conceptualized as multidimensionality. Recently, more formal descriptions of DIF as multidimensionality have become available in the item response theory literature. This approach assumes that DIF is not a difference in the item parameters of two groups; rather, it is a shift in the distribution of ability along a secondary trait that influences the probability of a correct item response. That is, one group is relatively more able on an ability such as test-wiseness. The parameters of the secondary distribution are confounded with item parameters by unidimensional DIF detection models, and this manifests as differences between estimated item parameters. However, DIF is confounded with impact in multidimensional tests, which may be a serious limitation of unidimensional detection methods in some situations. In the multidimensional approach, DIF is considered to be a function of the educational histories of the examinees. Thus, a better tool for understanding DIF may be provided through structural modeling with external variables that describe background and schooling experience. Index terms: differential item functioning, factor analysis, IRT, item bias, LISREL, multidimensionality.